The present invention relates to a method of determining a current position of an output element that is connected to an electric stepper motor. The present invention further relates to a control unit to execute the method.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Electric motors, such as stepper motors for example, have a rotor which is moved by a minimal angle or respective step size incrementally by a rotating electromagnetic field. Stepper motors find application, for example, for controlling components, i.e. output elements in a vehicle, which require precise control, i.e. control of very small control steps. Stepper motors can be used, for example, for controlling throttle valves which, in turn, control power output of an internal combustion engine.
To determine a position of the rotor of the stepper motor, the use of a sensor for position feedback, i.e. a rotor position sensor, such as, e.g., an encoder or rotary encoder, is possible because of the fact that stepper motors are electrically commutated motors, i.e. motors with a specific step size.
When the rotor of a stepper motor is excessively accelerated or decelerated, a so-called loss of step may be caused, which means that the rotor is unable to track a rotary field, generated when current is applied to various coils. Thus, when the rotor of the stepper motor is excessively accelerated or decelerated, single steps, i.e. revolutions caused by applying current to the respective coils, are skipped so that the position of the rotor can no longer be accurately ascertained.
A loss of step may be prevented, for example when using a so-called rotor position sensor, such as, e.g., an incremental encoder or absolute value encoder, which measures a rotational orientation or position of the rotor of the stepper motor. Thus, in the presence of a loss of step, operation of the stepper motor can be readjusted, using the rotor position sensor and the stepper motor can be operated up to its capacity.
Oftentimes, electric stepper motors are used in combination with a gear stage, like, e.g., a gear to drive an output element. The stepper motor includes hereby a rotor position sensor for optionally correcting operation of the stepper motor. The output element typically includes an absolute position sensor, e.g. like a Hall sensor, to meet legal requirements and to provide an absolute position signal of a current position of the output element.
It would be desirable and advantageous to address prior art shortcomings.